Plastic mulches provide many positive advantages for the user, such as increased yields, earlier maturing crops, crops of higher quality, enhanced insect management, and weed control.
Plastic mulches have been used commercially for the production of vegetables since the early 1960's, and their usage is still increasing throughout the world. Plastic mulches provide many positive advantages for the user, such as increased yields, earlier maturing crops, crops of higher quality, enhanced insect management, and weed control. They also allow other components, such as drip irrigation, to achieve maximum efficiency. Although a variety of vegetables can be grown successfully using plastic mulches, muskmelons, tomatoes, peppers, cucumbers, squash, eggplant, watermelons, and okra have shown the most significant responses. The production of strawberries and cut flowers, like vegetables is greatly improved by the use of plasticulture. The selection of which mulch type to use will depend on factors such as the crop to be grown, season of the year, whether double or triple cropping is contemplated, and if insect management is desired.
Much of the early work on the use of plastic mulches for vegetable production was to define the impact that differently colored mulches had on soil and air temperatures, moisture retention, and vegetable yields. Based on this work three main colors of black, clear, and white predominate commercial vegetable production today, although white has been replaced largely by a co-extruded white-on-black. Plastic mulches directly impact the microclimate around the plant by modifying the radiation budget (absorbitivity vs. reflectivity) of the surface and decreasing the soil water loss. The color of a mulch largely determines its energy-radiating behavior and its influence on the microclimate around a vegetable plant. Color affects the surface temperature of the mulch and the underlying soil temperature.
Another important factor is the degree of contact between the mulch and soil or by not being taut, often quantified as a thermal contact resistance, will greatly influence the performance of a mulch. If an air space is created between the plastic mulch and the soil by a rough soil surface, soil warming can be less effective than would be expected from a particular mulch. The soil temperature under a plastic mulch depends on the thermal properties (reflectivity, absorbitivity, or transmittancy) of a particular material in relation to incoming solar radiation. Black plastic mulch, the predominate color used in vegetable production is an opaque blackbody absorber and radiator. Black mulch absorbs most ultra-violet (UV), visible, and infrared wavelengths (IR) of incoming solar radiation and re-radiates absorbed energy in the form of thermal radiation or long-wavelength infrared radiation. Much of the solar energy absorbed by black plastic mulch is lost to the atmosphere through radiation and forced convection. The efficiency with which black mulch increases soil temperature can be improved by optimizing conditions for transferring heat from the mulch to the soil. Because thermal conductivity of the soil is high relative to that of air, much of the energy absorbed by black plastic can be transferred to the soil by conduction if contact is good between the plastic mulch and the soil surface. Soil temperatures under black plastic mulch during the daytime are generally 5° F higher at a 2-inch depth and 3° F higher at a 4-inch depth compared to those that of bare soil.
In contrast, clear plastic mulch absorbs little solar radiation but transmits 85% to 95%, with relative transmission depending on the thickness and degree of opacity of the polyethylene. The under surface of clear plastic mulch usually is covered with condensed water droplets. This water is transparent to incoming shortwave radiation but is opaque to outgoing longwave infrared radiation, so much of the heat lost to the atmosphere from a bare soil by infrared radiation is retained by clear plastic mulch. Thus, daytime soil temperatures under clear plastic mulch are generally 8 to 14° F higher at a 2-inch depth and 6 to 9° F higher at a 4-inch depth compared to those of bare soil. Clear plastic mulches generally are used in the cooler regions of the United States, such as the New England states. Using clear plastic mulch will require the use of a herbicide, soil fumigant, or solarization to control weeds.
White, coextruded white-on-black or silver reflecting mulches can result in a slight decrease in soil temperature -2° F at 1-inch depth or -0.7° F at a 4-inch depth compared to bare soil, because they reflect back into the plant canopy most of the incoming solar radiation. These mulches can be used to establish a crop when soil temperatures are high and any reduction in soil temperatures is beneficial. Depending on the degree of opacity of the white mulch, it may require the use of a fumigant or herbicide because of the potential weed growth. Another family of mulches includes the wave-length-selective or photoselective mulches, which selectively transmit radiation in some regions of the electromagnetic spectrum but not in the photosynthetic region. These mulches absorb protosynthetically active radiation (PAR) and transmit solar infrared radiation (IR), providing a compromise intermediate between black and clear mulch in terms of increasing soil temperature. The color of these mulches can be blue-green (IRT-76, AEP Industries Inc., Moonachie, N.J., or Climagro, Leco Industries, Inc., Quebec, Canada) or brown (Polyon-Barkai, Poly West, Encinitas, Calif.) These mulches warm up the soil like clear mulch but without the accompanying weed problem. An above-ground spectral response exists in addition to the response t o elevated soil temperatures, and may be physio-chemical (e.g. phytochrome regulation) or radiative (e.g., increasing or decreasing the heat load on the foliage). For example, in a pepper canopy, twice as much reflected photosynthetically active radiation (PAR) was measured above clear plastic mulch than above black plastic or bare soil. Although both red and black plastics raised soil temperatures similarly, higher early yields and less foliage were observed in plants grown on red plastic. Both red and black mulches reflected about the same amount of PAR, but red plastic increased the ratio of red:far-red wavelenghts (R:FR) in the reflected light. The R:FR ratio and the amount of blue light reflected toward the canopy apparently are critical. In turnips, blue and green mulches induced longer leaves and higher shoot:root ratios than white mulch. The R:FR ratio reflected from white plastic is lower than that of sunlight.
Additional colors that are being investigated currently are red, blue, yellow, gray, and orange, which have distinct optical characteristics and thus reflect different radiation patterns into the canopy of a crop, thereby affecting plant growth and development. This light reflectivity can affect not only crop growth but also insect response to the plants grown on the mulch. Yellow, red, and blue mulches increased green peach aphid populations, and the yellow mulch, which attracted increased numbers of striped and spotted cucumber beetles and Colorado potato beetles. Yellow has long been used in greenhouses to monitor the population of insects. Mulches with a printed silver surface color have been shown to repel certain aphid species and reduce or delay the incidence of aphid-borne viruses in summer squash. Similar to a white mulch, the degree of opacity of a gray mulch may require a herbicide or fumigant to be used to prevent weed growth. Some of these colored mulches (blue and red) had a dramatic impact on the soil temperatures, raising soil temperatures to 167 and 168° F, respectively, at the 2-inch depth when the ambient air temperature was 104° F.
Red plastic mulch was actually not introduced as mulch. Rather, its developers (USDA’s plant physiologist Michael J. Kasperbauer and Clemson University nematologist Bruce Fornum) found that it increased yields up to 20%. It was first recommended to commercial farmers, rather than home gardeners, who could benefit from higher production on a larger scale.
Along the way, gardeners began referring to red plastic tomato mulch as a mulching option.
While the product is not technically mulch, it’s increasingly used as one. Red plastic mulch (also known as Selective Reflecting Mulch, or SRM for short) is similar to black plastic mulch in that it warms the soil, prevents erosion, and retains moisture. It’s thinner than most garden plastic, allowing more light (and sometimes weeds) through.
But red plastic mulch’s touted strength is in its ability to reflect certain red shades of light back into the plant, accelerating fruit production and increasing yield.
How red plastic mulch works
If you’re a die-hard tomato grower, you’ve probably heard about using red plastic mulch to improve your yield and get your tomatoes to ripen earlier. Some early testing did show a considerable benefit from mulching with a specific shade of red plastic. But like all things scientific in the garden, there are still a lot of variables that prevent the results from being conclusive.
How does red mulch work?
Plastic mulches have been used commercially for the production of vegetables since the early 1960's, and their usage is still increasing throughout the world. Plastic mulches provide many positive advantages for the user, such as increased yields, earlier maturing crops, crops of higher quality, enhanced insect management, and weed control. They also allow other components, such as drip irrigation, to achieve maximum efficiency. Although a variety of vegetables can be grown successfully using plastic mulches, muskmelons, tomatoes, peppers, cucumbers, squash, eggplant, watermelons, and okra have shown the most significant responses. The production of strawberries and cut flowers, like vegetables is greatly improved by the use of plasticulture. The selection of which mulch type to use will depend on factors such as the crop to be grown, season of the year, whether double or triple cropping is contemplated, and if insect management is desired.
Much of the early work on the use of plastic mulches for vegetable production was to define the impact that differently colored mulches had on soil and air temperatures, moisture retention, and vegetable yields. Based on this work three main colors of black, clear, and white predominate commercial vegetable production today, although white has been replaced largely by a co-extruded white-on-black. Plastic mulches directly impact the microclimate around the plant by modifying the radiation budget (absorbitivity vs. reflectivity) of the surface and decreasing the soil water loss. The color of a mulch largely determines its energy-radiating behavior and its influence on the microclimate around a vegetable plant. Color affects the surface temperature of the mulch and the underlying soil temperature.
Another important factor is the degree of contact between the mulch and soil or by not being taut, often quantified as a thermal contact resistance, will greatly influence the performance of a mulch. If an air space is created between the plastic mulch and the soil by a rough soil surface, soil warming can be less effective than would be expected from a particular mulch. The soil temperature under a plastic mulch depends on the thermal properties (reflectivity, absorbitivity, or transmittancy) of a particular material in relation to incoming solar radiation. Black plastic mulch, the predominate color used in vegetable production is an opaque blackbody absorber and radiator. Black mulch absorbs most ultra-violet (UV), visible, and infrared wavelengths (IR) of incoming solar radiation and re-radiates absorbed energy in the form of thermal radiation or long-wavelength infrared radiation. Much of the solar energy absorbed by black plastic mulch is lost to the atmosphere through radiation and forced convection. The efficiency with which black mulch increases soil temperature can be improved by optimizing conditions for transferring heat from the mulch to the soil. Because thermal conductivity of the soil is high relative to that of air, much of the energy absorbed by black plastic can be transferred to the soil by conduction if contact is good between the plastic mulch and the soil surface. Soil temperatures under black plastic mulch during the daytime are generally 5° F higher at a 2-inch depth and 3° F higher at a 4-inch depth compared to those that of bare soil.
In contrast, clear plastic mulch absorbs little solar radiation but transmits 85% to 95%, with relative transmission depending on the thickness and degree of opacity of the polyethylene. The under surface of clear plastic mulch usually is covered with condensed water droplets. This water is transparent to incoming shortwave radiation but is opaque to outgoing longwave infrared radiation, so much of the heat lost to the atmosphere from a bare soil by infrared radiation is retained by clear plastic mulch. Thus, daytime soil temperatures under clear plastic mulch are generally 8 to 14° F higher at a 2-inch depth and 6 to 9° F higher at a 4-inch depth compared to those of bare soil. Clear plastic mulches generally are used in the cooler regions of the United States, such as the New England states. Using clear plastic mulch will require the use of a herbicide, soil fumigant, or solarization to control weeds.
White, coextruded white-on-black or silver reflecting mulches can result in a slight decrease in soil temperature -2° F at 1-inch depth or -0.7° F at a 4-inch depth compared to bare soil, because they reflect back into the plant canopy most of the incoming solar radiation. These mulches can be used to establish a crop when soil temperatures are high and any reduction in soil temperatures is beneficial. Depending on the degree of opacity of the white mulch, it may require the use of a fumigant or herbicide because of the potential weed growth. Another family of mulches includes the wave-length-selective or photoselective mulches, which selectively transmit radiation in some regions of the electromagnetic spectrum but not in the photosynthetic region. These mulches absorb protosynthetically active radiation (PAR) and transmit solar infrared radiation (IR), providing a compromise intermediate between black and clear mulch in terms of increasing soil temperature. The color of these mulches can be blue-green (IRT-76, AEP Industries Inc., Moonachie, N.J., or Climagro, Leco Industries, Inc., Quebec, Canada) or brown (Polyon-Barkai, Poly West, Encinitas, Calif.) These mulches warm up the soil like clear mulch but without the accompanying weed problem. An above-ground spectral response exists in addition to the response t o elevated soil temperatures, and may be physio-chemical (e.g. phytochrome regulation) or radiative (e.g., increasing or decreasing the heat load on the foliage). For example, in a pepper canopy, twice as much reflected photosynthetically active radiation (PAR) was measured above clear plastic mulch than above black plastic or bare soil. Although both red and black plastics raised soil temperatures similarly, higher early yields and less foliage were observed in plants grown on red plastic. Both red and black mulches reflected about the same amount of PAR, but red plastic increased the ratio of red:far-red wavelenghts (R:FR) in the reflected light. The R:FR ratio and the amount of blue light reflected toward the canopy apparently are critical. In turnips, blue and green mulches induced longer leaves and higher shoot:root ratios than white mulch. The R:FR ratio reflected from white plastic is lower than that of sunlight.
Additional colors that are being investigated currently are red, blue, yellow, gray, and orange, which have distinct optical characteristics and thus reflect different radiation patterns into the canopy of a crop, thereby affecting plant growth and development. This light reflectivity can affect not only crop growth but also insect response to the plants grown on the mulch. Yellow, red, and blue mulches increased green peach aphid populations, and the yellow mulch, which attracted increased numbers of striped and spotted cucumber beetles and Colorado potato beetles. Yellow has long been used in greenhouses to monitor the population of insects. Mulches with a printed silver surface color have been shown to repel certain aphid species and reduce or delay the incidence of aphid-borne viruses in summer squash. Similar to a white mulch, the degree of opacity of a gray mulch may require a herbicide or fumigant to be used to prevent weed growth. Some of these colored mulches (blue and red) had a dramatic impact on the soil temperatures, raising soil temperatures to 167 and 168° F, respectively, at the 2-inch depth when the ambient air temperature was 104° F.
Using red plastic mulch
While the product is not technically mulch, it’s increasingly used as one. Red plastic mulch (also known as Selective Reflecting Mulch, or SRM for short) is similar to black plastic mulch in that it warms the soil, prevents erosion, and retains moisture. It’s thinner than most garden plastic, allowing more light (and sometimes weeds) through.
But red plastic mulch’s touted strength is in its ability to reflect certain red shades of light back into the plant, accelerating fruit production and increasing yield.
How red plastic mulch works
Proteins: Their job is to regulate plant growth and development.
Phytochromes react differently to different spectra of light. Specifically, when far-red light wavelengths from the plastic bounce back up to tomato plants, the phytochromes tell the tomato fruit to grow more and faster.
That’s why gardeners say setting out red plastic mulch around their tomato plants makes their tomatoes ripen faster and produce bigger fruit.
Naturally, the amount of light reflected depends on growing conditions. Also, the plastic color must be specific. In other words, just any color of red plastic may not give good results, but rather those which have been developed and tested for this purpose. Recent studies suggest that other colors of plastic may offer greater yields than black, too, particularly blue plastic.
Bonus benefit: red tomato mulch deters nematodes
Additional studies have documented that red plastic mulch also reduces nematode damage. Nematodes (specifically, root knot nematodes) are worms that feed on a tomato’s root system. They are a particular problem in areas where the ground does not freeze, because they continue to live in the soil all year long.
A study by the USDA’s Agricultural Research Service found that red plastic mulch suppresses root nematode damage to tomatoes because the light reflection keeps more of the plant’s growth above ground. The plant’s energy goes into developing fruit and foliage, rather than roots. Nematodes feed on roots. The far-red light reflection to the above-ground plant draws away nutrients from the roots – and nutrients away from the nematodes. Fewer roots mean less food for nematodes. Less food = fewer nematodes.
Phytochromes react differently to different spectra of light. Specifically, when far-red light wavelengths from the plastic bounce back up to tomato plants, the phytochromes tell the tomato fruit to grow more and faster.
That’s why gardeners say setting out red plastic mulch around their tomato plants makes their tomatoes ripen faster and produce bigger fruit.
Naturally, the amount of light reflected depends on growing conditions. Also, the plastic color must be specific. In other words, just any color of red plastic may not give good results, but rather those which have been developed and tested for this purpose. Recent studies suggest that other colors of plastic may offer greater yields than black, too, particularly blue plastic.
Bonus benefit: red tomato mulch deters nematodes
Additional studies have documented that red plastic mulch also reduces nematode damage. Nematodes (specifically, root knot nematodes) are worms that feed on a tomato’s root system. They are a particular problem in areas where the ground does not freeze, because they continue to live in the soil all year long.
A study by the USDA’s Agricultural Research Service found that red plastic mulch suppresses root nematode damage to tomatoes because the light reflection keeps more of the plant’s growth above ground. The plant’s energy goes into developing fruit and foliage, rather than roots. Nematodes feed on roots. The far-red light reflection to the above-ground plant draws away nutrients from the roots – and nutrients away from the nematodes. Fewer roots mean less food for nematodes. Less food = fewer nematodes.
Red tomato mulch options
Red plastic is available to home gardeners in squares and in rolls. Squares are well suited for container tomatoes or for gardens with just a few tomato plants. If you have a larger number of tomatoes, rolls are the more economical choice.
Advantages of red plastic mulch
More than four out of five gardeners who use red tomato mulch would recommend it to a friend. Even so, there are some cautions.
Red plastic is available to home gardeners in squares and in rolls. Squares are well suited for container tomatoes or for gardens with just a few tomato plants. If you have a larger number of tomatoes, rolls are the more economical choice.
Advantages of red plastic mulch
- Bigger tomatoes
- More tomatoes
- Warms the soil
- Manageable for the home gardener, especially where smaller coverage needed
- Retains moisture in pots, a perennial problem
- Allow several feet of the red showing on either side of the plant or row.
- Use determinate tomato plants -- their finite size means they may maximize the amount of light reflected from the plastic.
- Run a drip hose underneath plastic, because the plastic forms a strong barrier. Water tends to pool on plastic surface even though it eventually penetrates.
More than four out of five gardeners who use red tomato mulch would recommend it to a friend. Even so, there are some cautions.
- Plan a watering method. The red tomato mulch offered by Gardener’s Supply has extra holes (“micro-perforations”) to allow water and air easily into the soil, but they’re tiny. Water can pool on the plastic in dips and valleys and stagnate. Some gardeners run drip hoses beneath the plastic to make sure plants get adequate water. (Gardeners who use black plastic do this too.)
- Handle carefully. Red plastic is thin and can tear easily. On average, it lasts two or three seasons. But there have been gardeners who handle it carefully and have used it up to 7 years.
- Replace it. When the plants get so big that no direct light hits the red plastic, switch it out and put down another type of mulch for the remainder of the season. You’ll save wear and tear on the red plastic for next season.
Does red plastic mulch help to ripen tomatoes faster?
How does red mulch work?
The idea behind using red mulch is that it reflects and bounces far-red light wavelengths back up into your tomato plants. These red light wavelengths stimulate the growth of tomato plants through a reaction with a specific pigment in the tomato plants (phytochrome). This seems to be true, but it also seems to be dependent on the amount of sunlight and maybe even temperature - something very much out of our control.
Whether red mulch means the end of late season green tomatoes remains to be seen. However, colored mulches in general appear to do a better job of warming the soil than black plastic, which at least gives your plants a head start.
Montana State University did a 2 year study that found silver and red mulches did speed ripening in tomatoes and that black mulch was not much better than no mulch at all. At Penn State, they found that blue mulch did an even better job of increasing tomato yields. However none of the colored mulches was as good as black plastic for suppressing weeds.
Whether red mulch means the end of late season green tomatoes remains to be seen. However, colored mulches in general appear to do a better job of warming the soil than black plastic, which at least gives your plants a head start.
Montana State University did a 2 year study that found silver and red mulches did speed ripening in tomatoes and that black mulch was not much better than no mulch at all. At Penn State, they found that blue mulch did an even better job of increasing tomato yields. However none of the colored mulches was as good as black plastic for suppressing weeds.
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What Does This Mean to You, as a Tomato Grower?
What Does This Mean to You, as a Tomato Grower?
It means that you can do your own experiments with colored mulches and see if it makes a difference in your garden. Red mulch is just beginning to be readily available on the market. You might even find it in your local garden center. If you’re determined to grow the more and better tasting tomatoes each year, red mulch is worth a try.
Please click here to view the report of Penn State Extension on coloured plastic mulch.
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